The gypsy moth, Lymantria dispar L., is a major forest pest in the northeastern United States, and spot infestations appear elsewhere. Monitoring existing gypsy moth populations and detecting new populations is achieved by utilizing traps baited with polymeric, controlled-release dispensers containing the sex attractant pheromone, (7R,8S)-cis-7,8-epoxy-2-methyloctadecane, or (+)-disparlure, wherein male moths are lured into the traps by the pheromone. When such lures are used in conjunction with insecticides, the moths are killed inside the traps.
The pheromone, disparlure, was identified, synthesized and reported in the literature by Bierl et al. (Science, 170:87-89, 1970). The optical character of the pheromone structure was later determined to be the (+)-enantiomer by Yamada et al. (J. Insect Physiol, 22:755-761, 1976). The pheromone has since been used in traps for the detection of gypsy moth populations and is commercially available. Its relatively high cost, however, dictates that the dispenser utilized must deliver nearly all of its pheromone content over one entire flight season of the moth. Otherwise, residual pheromone in the dispenser at the end of the season is wasted when the dispenser is discarded. It is equally important that the supply of the pheromone not be depleted before the end of the flight season.
In these types of traps, therefore, one must regulate the amount of pheromone released from the dispenser in the trap so that there is a balance between a rate of release sufficient to lure the moth into the trap and the length of time that the trap must remain attractive (one flight period). The dispenser must not deliver the pheromone so rapidly that the trap does not provide adequate luring capabilities over the entire flight period of the moth.
The pheromone (+)-disparlure is sufficiently volatile that the compound must be formulated in a controlled-release dispenser in order to prolong its evaporation and thus extend the duration of its effectiveness. Since the early 1980's, a dose of approximately 500 .mu.g of (+)-disparlure in a plastic laminate dispenser (Hercon Environmental Co., Emigsville, Pa.) has been used to bait a standard milk-carton gypsy moth trap used in all Federal and State detection programs (Schwalbe, C. P. in Technical Bull. 1584, USDA, Washington, D.C., pp. 542-549, 1981, Doane and McManus, ed.).
Normally, in assembling such traps, a twist tie is suspended within the trap by stapling it to the apex of the trap. The conventional pheromone dispenser is attached to the twist tie while an insecticide-impregnated strip is similarly attached near the dispenser. This is a labor intensive and tedious process since approximately 300,000 traps are required nationally each year.
Other commercial dispenser systems have been evaluated, but none has proven as effective as the plastic laminate device. Other commercial designs have included rubber septa, polyethylene tubes, silicone rubber, semi-permeable membranes over a reservoir and thermoset matrices.
A polyvinylchloride (PVC) pellet has also been tested which contains either 1250 or 2500 .mu.g of (+)-disparlure. Even at these relatively high doses, the release rates are initially low, i.e. only 57 and 97 ng/hr, respectively, as compared to the release rate (ca. 130 ng/hr) given off by the standard laminate containing only 500 .mu.g of pheromone. The low release rates per quantity of pheromone in the PVC pellets result from a low pheromone to polymer ratio and from a low surface area (ca. 75 mm.sup.2) for evaporation of the (+)-disparlure. After 16 weeks of greenhouse aging, which is equivalent to at least two flight seasons, these PVC pellets still contain at least 75% of their initial dose of pheromone. Thus, most of the pheromone is wasted in this type of dispenser since the planned use is for only one season.
PVC was used as a matrix for dispensing an insect pheromone by Fitzgerald et al. (Environ. Entomol., 2:609-610, 1973) when high quantities (1-20% by weight) of the cabbage looper pheromone were incorporated in 70-mm long PVC cylinders. Since then, other pheromones were formulated in PVC forms, including the pheromone of the tobacco budworm, the boll weevil and the spruce budworm. The forms of the previously described PVC dispensers were a solid mass such as a slab, disc, rod, etc. which had a limited surface area for evaporation and release (See Weatherson, I. in Insect Pheromones in Plant Protection, Jutsum and Gordon, eds., John Wiley & Sons, New York, pp. 249-280, 1989, a review article).
The dose of pheromone per dispenser was substantially higher (2000-10,000 .mu.g or more) in previously reported PVC applications than that used for the laminate dispenser (500 .mu.g). Only PVC pellets designed for monitoring spruce budworm contained a relatively low dose (0.03% by weight), but those dispensers were shown to be unsatisfactory in field performance. The volatility of pheromones incorporated in previous. PVC applications was higher and their cost substantially lower than that of (+)-disparlure, thus obviating a need to develop a PVC dispenser having an increased surface area for increasing its release rate or having an optimal efficiency for delivering its total pheromone content over a particular period of time. Thus, the need exists for a dispenser which will give a sufficient pheromone release rate while at the same time prolonging the rate of release such that pheromone is released over the entire insect flight period in a given area and delivers essentially all of the pheromone contained within the dispenser over that period.